The present invention relates to a reverse gear synchronizer in a motor vehicle manual transmission.
The typical motor vehicle manual transmission has an input shaft, a splined main shaft (output shaft) and a counter shaft. The input shaft, which is connected to the engine of the vehicle via a clutch, has a gear attached to one end for driving a corresponding gear located on the counter shaft. The input shaft gear and the corresponding counter shaft gear are in constant engagement so that as the input shaft rotates, the input shaft gear likewise rotates thus rotating both the corresponding gear on the counter shaft.
Usually, the counter shaft is provided with several fixed driver gears which rotate at a speed constant with the counter shaft. Each fixed driver gear is in constant engagement with a corresponding driven gear located on the main shaft so that as each fixed driver gear rotates with the counter shaft, each corresponding driven gear is rotated in the opposite direction. Each engaged driver gear and corresponding driven gear define a forward gear ratio, usually referred to as first gear, second gear, third gear, forth gear, and so on. Each gear ratio is used to increase the torque transmitted from the engine to the wheels by varying degrees.
Typically, one shift collar is located between adjacent driven gears and is splined for fixed rotation relative to the main shaft. Further, each shift collar is axially slidable along the main shaft for engagement with a corresponding adjacent driven gear. Each driven gear is free to rotate independently of the main shaft until locked into position by a shift collar, as will be explained further.
Each driven gear is typically provided with large teeth known as "dogs", located on a side of the gear for engagement with similar dogs located on the adjacent shift collar. The engagement between the shift collar, which is rotating with the main shaft, and the selected driven gear locks the gear to the shaft so that power from the engine is transmitted through the selected driven gear and the main shaft to the wheels of the vehicle.
The driver of the motor vehicle shifts into a desired gear ratio using a shift lever and attached shift forks which engage with one or more of the shift collars. Depending upon the gear ratio selected, the shift lever moves a shift fork or forks and a corresponding engaged shift collar or collars axially along the main shaft, until the collar or collars engage an adjacent driven gear, locking the driven gear to the main shaft.
To achieve a smooth engagement between the dogs of the collar and the gear, synchronizing systems have been developed which allow the two sets of dogs to reach the same speed before they are engaged. Once their speeds are synchronized, the two sets of dogs can mesh smoothly.
The typical vehicle transmission is also provided with a reverse gear system so that the vehicle may be moved in a reverse direction. The reverse gear system usually comprises a fixed reverse driver gear located on the counter shaft, and a corresponding driven gear located on the main shaft. In order to reverse the direction of rotation of the main shaft relative to the forward gear direction, a reverse idler gear must be provided between the reverse driver gear and the reverse driven gear. When all three gears of the reverse gear system are in engagement, the reverse driver gear rotates the idler gear, which in turn rotates the reverse driven gear.
Various methods of engaging the reverse gears together have been developed as are disclosed, for example, in U.S. Pat. No. 3,245,278; U.S. Pat. No. 3,478,615; U.S. Pat. No. 3,745,847; U.S. Pat. No. 2,753,728; U.S. Pat. No. 4,856,361; U.S. Pat. No. 4,370,896; U.S. Pat. No. 4,257,284; U.S. Pat. No. 4,531,418; and U.S. Pat. No. 4,263,815. For example, it is known to spline or key all three reverse gears (i.e., the reverse driven gear, the reverse driver gear and the idler gear) to their respective shafts, so that as an individual reverse gear rotates, the corresponding shaft likewise rotates. In order to shift into reverse gear using this configuration, at least one of the three reverse gears is typically slidable along its respective splined shaft to engage the other reverse gears. In this manner, the teeth of one reverse gear, for example the idler gear, slide into engagement with the teeth of the reverse driver gear and the teeth of the reverse driven gear, allowing the transfer of power from the engine to the wheels of the vehicle.
A sliding reverse gear system has numerous associated problems. First, the reverse gears must be straight spur gears. A straight spur gear has straight teeth cut parallel to its axis of rotation and is needed for the teeth of two gears to slidingly engage with each other. However, straight spur gears tend to be noisy in operation. A possible solution for reducing gear noise is to use helical type gears which have curved teeth cut at an angle to the axis of rotation, and allows for at least two teeth on each gear to be in constant meshing engagement. This kind of meshing action provides for smoother, quieter movement, and is used for the forward gears. However, each helical gear must be in constant meshing engagement with its corresponding helical gear, because helical gears cannot be axially slid into engagement with each other.
Second, during the sliding process of a straight gear, it is possible that the two straight gears do not fully engage. This condition would place added stresses on the portions of the teeth that are engaged, resulting in possible tooth breakage. Further, partially engaged gears are subject to torsional forces, contributing to gear jump out.
Third, the sliding gear must slide at least a distance equal to its width to either fully engage or fully disengage the corresponding gear. This requires more room in the transmission housing than a gear and collar system. Further, the sliding gear is moved via the shift lever, which is moved by the driver. Because the shift lever must move the sliding gear a greater distance, the shift lever must also move a corresponding greater distance. This extra movement of the shift lever thus requires extra space within the interior of the vehicle.
It is also known to divide the reverse idler gear into a double idler gear, as is disclosed in U.S. Pat. No. 4,836,041. In this system, the reverse idler gear comprises two separate axially aligned gears. One of the gears is in constant meshing engagement with a corresponding driven gear and the other gear is in constant meshing engagement with a coupling sleeve which, in turn, constantly meshes with the main shaft. When not engaged together, the two idler gears are free to rotate independently of one another. To engage the two gears together, one of the two gears is axially slid towards the other gear. Eventually, the two gears come into contact, locking the gears together. A smooth engagement between the two idler gears is ensured by a synchronizer arranged therebetween. However, this system has all of the aforementioned disadvantages of a sliding gear. Furthermore, an additional drive gear has to be cut on the countershaft. Also, both gears require a complex configuration.
Another known system uses a double idler gear system as described above, but the two adjacent reverse idler gears are axially fixed along the shaft. Each gear is provided with at least one dog on adjacent sides. A sleeve is provided that is axially slidable along the shaft to engage the dogs of the two gears together, so that the two gears rotate as a single gear. This system has the advantage of being able to use helical gears; it lacks, however, a synchronizing system.
In order to shift into the reverse gear using a nonsynchronized reverse gear configuration, all of the reverse gears would typically need to be at a complete stop so that the teeth of the gears could slide into engagement. Stated alternately, the wheels of the vehicle would need to be motionless so that the main shaft and the connected driven reverse gear do not rotate. However, even when the vehicle is motionless, the counter shaft and the attached fixed gears may rotate due to inertia. Thus, unless the reverse gears are fully synchronized, the teeth or dogs of the reverse gears may grind before meshing.
In the reverse synchronizer disclosed in U.S. Pat. No. 4,856,361 a fixed gear on the countershaft drives an idler gear which, in turn, drives the reverse gear located on the main shaft. The reverse gear is provided with "dogs" and is free to rotate independently of the main shaft until it is engaged by the collar of the reverse synchronizer.
In still another prior art construction the reverse gear is affixed to the main shaft, the reverse idler gear rotates freely and the synchronizer is located on the countershaft.
It is a disadvantage of both last-named systems that they require excessive axial space.